As is well known, recently, with the rapid increase of the number of internal combustion engines, particularly in motor cars, pollution of atmospheric air by exhaust gas thereof has become a social problem. To solve this problem, various purifiers have been proposed. However, there have been many difficult Zproblems with these purifiers such as cost, purification capacity and mountability on the motorcar, etc.
Generally, in engines for a motor car, the mixing ratio of air and fuel supplied to each cylinder (hereinafter called the air fuel ratio) is designed so as to be substantially uniform, and in case of partial engine load, with an air fuel ratio set for minimum fuel consumption, the concentration of nitrogen oxide (hereinafter designated NOX) in the exhaust gas becomes high, and when the throttle is nearly full open, and the fuel ratio is set for maximum output, the concentration of carbon monoxide (hereinafter designated CO) and of hydrocarbon (hereinafter designated HC) are high. Therefore, the usual multicylinder engines have the disadvantages that the concentration of NOX, CO and HC for a fixed operating mode of exhaust gas measuring are high. However, as to CO and HC, when the engine is operated at a low number of revolutions the mixed gas is incompletely combusted in the cylinder due to the low temperature of the inner wall of the cylinder, etc., so that noxious exhaust gas containing combustible components such as CO, HC, etc. will be generated.
As to said CO and HC, as is well known, when they are efficiently burnt at a high temperature by supplying sufficient air, their concentration may be decreased, while as to NOX, the higher its combustion temperature becomes, the more the reaction of N.sub.2 + O.sub.2 .fwdarw.2NO - Q Kcal proceeds, thus increasing the concentration of NOX. Therefore, for the purpose of decreasing this NOX, it is necessary to lower engine efficiency by lowering the combustion temperature, and as one of its means of solution, an exhaust gas reflux system has been adopted, in which a part of the exhaust gas is sucked back into the intake. However, in such a system, since to combustion becomes unstable unless excess fuel is supplied, a mixed gas feeder for feeding an air-fuel mixture with a larger concentration of fuel than in the normal air-fuel ratio i.e. an over-concentration mixed gas, is operated at the same time as the exhaust gas reflux to supplement the supply of fuel for stabilizing the combustion. However, since it is necessary to control conditions so as to make the percentage of non-combustible gas nearly constant, including residual gas within the combustion chamber, there is the disadvantage that a device having a simple construction and low cash cannot be used for this system.